In the field of mobile communications, technological studies on a multimedia broadcast/multicast service (hereinafter referred to as “MBMS”) are being carried forward recently (e.g., see “3GPP TS 22.146 V6.0.0 (2002-06): 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Multimedia Broadcast/Multicast Service; Stage 1 (Release 6) June 2002”). A communication carried out in an MBMS is not a one-to-one (Point to Point: P-to-P) communication but one-to-multi (Point to Muiti: P-to-M) communication. That is, in the MBMS, one base station transmits the same data (e.g., music data and video image data, etc.) to a plurality of mobile stations simultaneously.
The MBMS has a broadcast mode and multicast mode. While the broadcast mode is a mode in which information is transmitted to all mobile stations as with current radio broadcasting, the multicast mode is a mode in which information is transmitted to only specific mobile stations affiliating a news group or other services.
Advantages in carrying out the MBMS include the following: That is, when each mobile station receives information transmitted from a base station through a streaming service, etc., using one channel, if the number of mobile stations requesting the information increases, the load on the radio channel increases. However, when the MBMS is used, even if the number of mobile stations increases, all those mobile stations receive the information using the same channel, and therefore it is possible to increase the number of mobile stations capable of receiving the information without increasing the load on the radio channel. Currently, distribution of traffic information, music distribution, news distribution at a station, distribution of live coverage of a sport event, etc., are considered as services available using the MBMS and providing these services at a transmission rate of approximately 8 to 256 kbps is under study.
In an MBMS, use of an S-CCPCH (Secondary Common Control Physical Channel) which is used in a current W-CDMA mobile communication scheme as a channel to transmit the same data to a plurality of mobile stations simultaneously is under study. The S-CCPCH is a downlink common channel and is used as a paging signal and for data transmission from a higher layer according to the current W-CDMA mobile communication scheme. Furthermore, the S-CCPCH does not perform transmit power control and transmits data with relatively large constant transmit power that can cover the entire cell (e.g., see “3GPP TS 25.211 V5.1.0 (2002-06): 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical channels and mapping of transport channels onto physical channels (FDD) (Release 5) June 2002, 5.3.3.4 Secondary Common Control Physical Channel (S-CCPCH”). This constant transmit power is instructed from a higher layer (control station) to the base station.
Here, as shown in FIG. 1, even if an MBMS is carried out, not only the S-CCPCH for the MBMS but also a DPCH (Dedicated Physical Channel) for carrying out normal speech communication and transmission of dedicated control information, etc., exists as with conventional cases between the base station and mobile station. The DPCH is an uplink/downlink bidirectional dedicated channel and the DPCH is a channel dedicatedly assigned to each mobile station as opposed to the S-CCPCH which is a channel commonly used for all mobile stations within the cell.
As described above, the transmit power of the S-CCPCH is transmit power which is so large and constant that it reaches the cell boundary irrespective of the positions of mobile stations located in the cell. For this reason, for a mobile station located near the base station, its reception quality becomes excessive, which results in waste. Furthermore, interference with other cells also increases causing a reduction of the subscriber capacity (system capacity) of the entire system. Furthermore, an upper limit (maximum transmit power at the base station) of total transmit power of all channels is predetermined for the base station, and therefore if the transmit power of the S-CCPCH is large as shown in FIG. 1, transmit power available for other channels such as the DPCH is relatively reduced and communication quality of a speech communication, etc., deteriorates.